The present exemplary embodiment relates to document processing systems and more particularly to improved process control patch measurement methods and print control systems. In the present disclosure, methods and systems are provided for print engine control in a document processing system. Electrophotographic laser printers, xerographic copiers, scanners, facsimile machines and other document processing systems are operable to print images and text onto printable media using imaging components operated according to print data. Electrostatic charge is initially distributed on an outer side surface of a photoreceptor or photographic member, which may be a rotating drum or a belt translated in a process direction in the printing engine. A customer image is optically projected on the charged photoreceptor surface using a raster image scanner leaving a latent image on the photoreceptor surface. The image is developed by the photoreceptor translating past a source of toner such that toner particles are drawn to the latent image via a carrier in order to create a visible image on the photoreceptor surface, and the toner image is then transferred to paper or other printable media and is fused to the media to create a printed document. Following the printing cycle, the photoreceptor is discharged before the photoreceptor is again charged for printing of the next customer image.
To ensure correct operation of the imaging components of a print engine over time, feedback control techniques are employed using feedback obtained from process control or test patches created on the photoreceptor to adjust the operation of the various imaging components of the printing engine. The control patch is a predefined pattern that is created on the photoreceptor surface between adjacent customer images and is sensed or measured as the photoreceptor moves past a sensor. Different patches may be created for different colors, with each patch including light, dark, and medium areas, where more than one sensor may be used to measure the differently colored control patches created on the photoreceptor. Feedback information obtained from the measured control patch is used to adjust operating parameters of the printing system to maintain image quality, such as toner concentration, the magnitude of the charge on the photoreceptor, the amount of exposure from the scanner, etc.
FIG. 12 shows a conventional process control patch implementation in which control patches 130 for each developer housing are created in an inter-document zone (IDZ) 122 between successive customer images 120 on a drum or belt type photoreceptor 110a that moves along a process direction P. These control patches 130 are sensed with toner patch sensors (not shown) and actuators are adjusted to maintain a three-point tonal response curve (TRC). However, the inventors have appreciated that shortfalls in system performance, such as reload degradation, can cause undesirable interactions between the control patches and the customer images. In this situation, the presence of the customer image may cause distortion in the control patch image, and thereby lead to an error in the measured process control feedback for the print engine. Likewise, the presence of control patches in the IDZ may cause a distortion in the customer image. Thus, there is a need for improved feedback control techniques and systems for print engine control.